[CentOS] CentOS 5.7 eth0, eth1 and arpwatch flip flops

Wed Dec 26 12:33:17 UTC 2012
Mike Burger <mburger at bubbanfriends.org>

Mike Burger

"It's always suicide-mission this, save-the-planet that. No one ever just
stops by to say 'hi' anymore." --Colonel Jack O'Neill, SG1

> Dear all,
> Has anyone experienced this whilst running DRBD over eth1 between two
> CentOS 5.7 servers?
> eth1 is a private IP address, unroutable. eth0 is the public address.
> CentOS will reply sometimes once every 3 days or every 14mins~ saying
> "My public IP is on eth1" to arp requests when it's not, it's eth0.
> This freezes traffic and causes issues. We've looked at arp*
> /etc/sysctl.conf etc. and arptables, but wondered if anyone had a
> recipe?
> We have this already which didn't help as it happened Christmas Eve
> too as we kept eth1 up for a few days to test:
> # For the dual interface - 06.12.12
> net.ipv4.conf.eth0.arp_filter = 1
> net.ipv4.conf.eth0.arp_ignore = 1
> net.ipv4.conf.eth1.arp_filter = 1
> net.ipv4.conf.eth1.arp_ignore = 1
> net.ipv4.conf.all.arp_filter = 1
> eth0 and eth1 are on the same VLAN/broadcast domain, but eth1 is a
> 169.xx and eth0 is a routable public IP.
> We're having to shut eth1 down and bring it up for sync at night.

To what type of equipment are your ethernet devices connected? Are they
both connected to the same device? I've seen some devices (particularly
2Wire) that do not like two interfaces from the same system connected to

You note that eth1 is on a 169.xx IP, and earlier in your email, you note
that it's non-routable. Perhaps that's not the wording you wished, to use,
or perhaps you meant that it's not routed out to the internet, however,
169.xxx.xxx.xxx is most certainly a "routable" IP block, as far as
internet standards go.

The only "non-routable" (i.e. reserved for private networks) IP blocks are: - - -

While certainly not an undertaking to be done lightly, you may wish to
renumber your internal network.